High mobility, n-type transparent thin-film transistors (TTFTs) with a zinc indium oxide (ZIO) channel layer are reported. Such devices are highly transparent with ∼ 85% optical transmission in the visible portion of the electromagnetic spectrum. ZIO TTFTs annealed at 600 °C operate in depletion-mode with threshold voltages −20 to −10 V and turn-on voltages ∼ 3 V less than the threshold voltage. These devices have excellent drain current saturation, peak incremental channel mobilities of 45–55 cm² V-¹ s-¹, drain current on-to-off ratios of ∼ 10⁶, and inverse subthreshold slopes of ∼ 0.8 V/decade. In contrast, ZIO TTFTs annealed at 300 °C typically operate in enhancement-mode with threshold voltages of 0–10 V and turn-on voltages 1–2 V less than the threshold voltage. These 300 °C devices exhibit excellent drain–current saturation, peak incremental channel mobilities of 10–30 cm² V-¹ s-¹, drain current on-to-off ratios of ∼ 10⁶, and inverse subthreshold slopes of ∼ 0.3 V/decade. ZIO TTFTs with the channel layer deposited near room temperature are also demonstrated. X-ray diffraction analysis indicates the channel layers of ZIO TTFTs to be amorphous for annealing temperatures up to 500 °C and polycrystalline at 600 °C. Low temperature processed ZIO is an example of a class of high performance TTFT channel materials involving amorphous oxides composed of heavy-metal cations with (n−1)d¹⁰ns⁰ (n ≥ 4) electronic configurations.